Recently, as recording systems distinct from the thermal transfer recording system using a thermal head, a conductive thermal transfer recording system, a laser thermal transfer recording system and an optical thermal conversion recording system have been proposed.
Generally, a conductive thermal transfer system, as shown in FIG. 10, comprises a conductive thermal transfer ink medium d wherein a conductive layer b and an ink layer c are laminated on an ohmic layer a in that order and a conductive transfer recording (stylus) head g wherein a plurality of electrodes f are provided on an insulating substrate e. By making the conductive transfer recording (stylus) head g and the return electrode h contact the ohmic heating layer a and applying a voltage in accordance with the image information to specified electrodes f of the recording head g, a current is selectively supplied between the electrodes f and the conductive layer b, by which the ohmic heating layer a disposed between them is heated and ink from the portions corresponding to the heated portions x of the ink layer is transferred to a recording medium i. This mechanism is described in documents such as "Tsuden netsu tensha houshiki ni yoru color video printer I" (Color Video Printer Using Conductive Thermal Transfer System I), Minoru Usui, Manabu Nishiwaki et al., a document prepared for Gazou Denshi Gakkai 93th research symposium, 1986, pp. 1-6, "Tsuden nessha kiroku heddo no kaihatsu" (Development of Conductive Thermal Transfer Recording Head), Hiroyuki Sawai, You Ishii et al., a document prepared for Gazou Denshi Gakkai 102th research symposium, 1988, pp. 25-30.
Compared to a thermal transfer recording system using a thermal head, this conductive thermal transfer system is capable of printing at high speed and with high quality on plain paper.
This system, however, has technical problems with high density printing of the order of 31.49606 dots/mm (800 dpi), which makes it difficult to make a recording head g in respect of its mechanical structure and increases the manufacturing cost significantly. Further, in this system, as conduction and heating are carried out by contacting the recording head g with the conductive thermal transfer ink medium i, the following problems tend to occur: a tendency to image degradation because of non-uniformity of the printed dot diameter in the axial direction of the print head, reduction of print quality caused by abrasion of electrodes and attachment of printing debris between electrodes, and the difficulty of increasing the printing speed in respect of heating efficiency.
The laser thermal transfer recording system, as shown in FIG. 11, comprises a laser thermal transfer ink medium m formed of a substrate i having an optical thermal conversion heating layer k of carbon black or the like and an ink layer l laminated thereon. If a laser light n corresponding to an image information is directed at the substrate i, the portion x of the heating layer k corresponding to the laser light is selectively heated, and the ink from the corresponding portion of the ink layer l is transferred to a recording medium i, and thus printing is carried out. This system is described in, for example, "Laser netsu tensha no kiroku tokusei V" (Characteristics of Thermal Transfer Printing by Laser Heating-Study of Recording Energy-), Mitsuru Irie and Takashi Kitamura, a thesis prepared for the Denshi Shasin Gakkai annual symposium "Japan Hard Copy '92", 1992, pp. 45-48.
Although the problems of the above described conductive thermal transfer system, particularly the problems with high density recording, are solved with this laser thermal transfer recording system, as it has a mechanism wherein the optical thermal conversion heating layer k on which the laser light n impinges generates heat itself and the heat is conveyed by thermal diffusion to the ink layer l, the recording sensitivity may be reduced by the expansion of the heating region, there may be loss and absorption of heat in the heating layer caused by the thermal diffusion, and increasing the printing speed is also difficult. This system is suitable for monochrome printing, but in color printing, recording sensitivity and recording speed have tendency to be reduced as described above.
The optical thermal conversion recording system, as shown in FIG. 12, comprises a heating rotatable drum s formed of the following layers laminated on a light-transmitting hollow cylinder o in this order: a transparent conductive layer p, a photoconductive layer q and a conductive layer r and between the transparent conductive layer p and the conductive layer r of which, a voltage is applied. The laser light n in accordance with image information is directed from inside the cylinder of the heating rotatable drum s, and selectively makes the photo-conductive layer q conductive, allowing a current to flow, and thus heating the portion x corresponding to the laser light. In this way, as in a thermal head, ink from the heat-sensitive transfer film t is transferred to a recording medium i which is transported between the heating rotatable drum s and the roller v with the heat-sensitive transfer film t therebetween. This system is described in, for example, Japanese unexamined patent publication No. Hei 4-14480 (1992). In the figure, x is the heating portion, u is a power source and v is a platen roller.
Compared to the conventional recording system using a thermal head or a conductive transfer recording head, as this optical thermal conversion recording system uses a heating rotatable drum s wherein the heat for image printing is generated by light radiation as a thermal head, there is less friction between the recording head and the heat-sensitive recording material, that is the ink medium t. Therefore, distortion when printing and reduction of printing quality caused by attachment of debris is prevented.
With this recording system, however, the thermal retention phenomenon of the recording head in accordance with the increase of printing speed is reduced, because the same portions are not heated repeatedly as in a conventional thermal head, but the following problems tend to occur: non-uniformity of print density which is caused by the thermal retention phenomenon in the heating rotatable drum (head) s caused by high speed or long-term printing, and the reason for which is also the lack of stability over time of the heating characteristics, non-uniformity of the printed dot diameter in the axial direction of the print head because heating and then printing is carried out by making the heating rotatable drum s, which is a recording head, contact the heat-sensitive recording material t as in a conventional thermal head, non-uniformity of print density caused by the loss or failure when conveying heat from the rotatable drum s to the heat-sensitive recording material t or debris on the drum surface, the difficulty of increasing the printing speed in respect of thermal conveying efficiency, and so forth.